After discovering a new antimicrobial peptide, cecropin from the silkworm larva as a result of a defense-mechanism in insects against invasion of microorganisms, peptides have begun to be recognized as important biologically active materials. Recent studies show that most of the higher living things accumulate in or secrete into their bodies antimicrobial peptides as a defense-mechanism against pathogens, independently from the immune system. More than 2,000 antimicrobial peptides have been discovered up to date. These peptides found in different species have different amino acid compositions, but the mechanisms of antimicrobial activity are similar to one another.
The most widely known antimicrobial peptides include cecropin, magainin, bombinin, defensin, tachyplesin and buforin. These antimicrobial peptides are composed of 17-24 amino acids, and have antimicrobial activity against Gram-negative and Gram-positive bacteria as well as protozoa and fungi. Some of these peptides show anti-cancer or anti-viral activity. Especially, magainin is a peptide with 23 amino acids separated from the epidermis of amphibians (Zasloff, M. (1987) Proc. Natl. Acad. Sci. USA, 84:5449-5453) and can act against human lung cancer cells as well as pathogens. Also, most of the antimicrobial peptides act and kill the target cells specifically and promptly, and exhibit activity spectrum against a wide range of microorganisms (Park, C. B., Kim, M. S. and Kim, S. C. (1996) Biochem. Biophys. Res. Comm. 218:408-413).
The above antimicrobial peptides                1. have strong antimicrobial activity against a wide variety of microorganisms,        2. are not toxic to human body since they do not destroy host cells, but act specifically against extraneous pathogens,        3. have little possibility to cause resistance since they show antimicrobial activity by totally different mechanisms from conventional antimicrobial drugs causing resistance,        4. can be mass produced by genetic modification since they do not undergo secondary modification such as glycosylation, and        5. have high commercial value in pharmaceutical and food industries since they are physico-chemically stable against heat, acid or alkali.        
The action mechanism of antimicrobial peptides reported up to now can be categorized into two, as follows;
First, most of the antimicrobial peptides have an action mechanism of destroying membrane potential by increasing cell membrane permeability and stopping the cellular metabolism. Currently, numerous research results are being reported on the biochemical and structural characteristics of the antimicrobial peptides exhibiting the above action mechanism.
Second, a small number of antimicrobial peptides are able to penetrate into microbial cells and strongly act against the microorganisms by combining with DNA or RNA and prohibiting transcription or translation, but the mechanism of this strong antimicrobial activity, is not being investigated. However, since antimicrobial drugs with new action mechanism are developed actively due to the emergence of microorganisms that are resistant to antimicrobial drugs, it is important to understand the action mechanism of the antimicrobial peptides that is able to penetrate into microbial cells and act against the microorganisms, and it is also important to develop these antimicrobial peptides.
The salient structural features known to be important in the activity of the antimicrobial peptides that is able to penetrate into microbial cells and act against the microorganisms include,                1. amphipathic helix        2. distribution of residues stabilizing the above helix,        3. distribution of basic residues,        4. distribution of hydrophobic residues,        5. dipole interaction between charged residues and amphipathic helix, and        6. salt-bridge between the residues with different charges.        
Noticing the above observations, the present inventors have perfected the present invention by synthesizing new antimicrobial peptides, having amino acid residues of these peptides substituted, added or deleted, and then selecting repeatedly the peptide analogs which is able to penetrate into microbial cells and act against the microorganisms.